My cb400t had the a simalar problem, no power after 5000 rpm. Theres actally two charging circuits for the capacitor in the cdi box. Theres an exiter coil and another coil in the stator down on the engine. The exiter coil on these bikes gets hot and can burn open. with no exiter coil circuit the ignition won´t build enough power to run at high speed. people mistake this as a carb problem. So check your exiter coil resistance should be i think 135 ohms its pink wire to engine ground. If you got that you may have a bad CDI. I make and sell them on ebay. Look for my it and my ID .dontasmebro